Continuing developments in radio and microwave frequency components (e.g. power transistors and monolithic microwave integrated circuits—MMICs) have resulted in the desirability of performing on-wafer load-pull measurements, to speed up development of such devices. A load-pull technique involves the provision of an arbitrary (controlled) impedance at a predetermined location (the reference plane) in a circuit, for example the location where a device under test (DUT) will be connected.
An analogous need exists for the provision of a signal generator with an arbitrary controlled output impedance at a reference plane (known as source-pull).
Existing techniques for implementing load-pull measurements are summarised and reviewed in, for example, “Comparison of active versus passive on-wafer load-pull characterization of microwave and mm-wave power devices” by J.-E. Muller and B. Gyselinckx, IEEE MTT-S Digest, 1994, pp. 1077-1080 (WE3F-40). These techniques can be classified as either:                Passive loads, such as slide-screw mechanical or solid-state tuners. These have many advantages, including low risk of oscillation, high power-handling capability and simple and precise controllability. However, they also have a severely limited maximum load reflection coefficient at the probe tip connected to the DUT, owing to power losses in the cables and connectors. This restricts the range of impedances that can be generated, and is a severe limitation for on-wafer characterization of power transistors.        Active loads, incorporating an amplifier and either a power splitter to provide two signal paths to drive both ports of a DUT, or a feedback arrangement including a single directional coupler or circulator. The two signal paths approach has a low risk of oscillation, but a power sweep in the non-linear regime of the DUT requires a complicated sequence of settings of adjustable components at each power level to keep the reflection coefficient constant. The feedback arrangement avoids this problem, but has a higher risk of oscillations, especially if a broadband amplifier is included in the feedback path, as well as other limitations.        